This invention relates generally to mechanical pencils and more particularly to a mechanical pencil which operates, each time it is subjected to a forward shake in the axial direction of the barrel or holder, to feed or project a pencil lead by a specific distance out through the writing end of the pencil.
Throughout this disclosure, the writing end of the pencil and the other end thereof are respectively referred to as the "front" and "rear" ends, and, correspondingly directions toward these ends are respectively referred to as "forward" and "rearward".
Known mechanical pencils have generally been of the rotating type or the "knocking", or knob-pressing, type. These types require troublesome manipulations for feeding out the lead for replenishment. For example, when the lead of a known mechanical pencil, held in a writer's hand for writing, becomes short and must be further projected forward, the lead-feeding procedure entails the steps of regrasping the pencil by one or two hands, rotating one part of the pencil or pushing a knob, and regrasping again the pencil for writing. These manipulative steps are extremely inefficient. In a known mechanical pencil, furthermore, the wedge-shaped part of a chuck for clamping the lead is pressed into a tightening ring by the elastic force of a front spring and is thereby forced by the wedge action to clamp and hold a piece of lead.
For this reason, unless this state of the lead held only by elastic force of the front spring is amply secure, that is, unless the initial lead holding force is amply great, the lead cannot be positively held since additional lead holding force cannot be obtained from the writing force, that is, the reaction force of the writing surface on the lead. Accordingly, the improvement of the chuck performance for assuring ample initial lead holding force has heretofore been a difficult problem. For this reason, a load on the front spring at the time of its installation of more than approximately 500 grams has been absolutely necessary, and even with this great installation load, problems have remained.
Furthermore, this installation load of the front spring has, in addition to its relationship to the initial lead holding force, a close relationship also to the balancing thereof with the pressing force of the eraser rubber arising from the use of the eraser rubber. As a result of our study directed toward the solutions of these problems, we have found that, by fabricating the chuck so that it has greater flexibility and improving the fabrication precision of the tightening ring, these problems can be overcome, and, moreover, by reducing the installation load of the front spring relative to those used heretofore, an initial lead holding force of ample magnitude can be obtained.
As a result of further study, we have succeeded, by further lowering the installation load of this front spring, in providing a mechanical pencil in which the lead can be progressively fed by merely utilizing the momentum of a weight member without carrying out rotating or "knocking" manipulation.
The reason for the necessity of a very low installation load of the front spring is that the required weight of the weight member whose momentum is utilized for feeding the lead increases as the installation load of the front spring increases, whereby a large load gives rise to a large total weight of the pencil, which would be highly undesirable.